A liquid crystal display device (LCD) includes liquid crystal twisted 90 degrees between a polarizer and an analyzer which have vertically polarization directions to each other. When applying no voltage, the linearly-polarized light passing the polarizer is locally rotated along with the direction of another liquid crystal alignment body and deflected in 90 degrees. As a result, the light is rotated and passed through an analyzer while passing a liquid crystal layer. When applying voltage, since the liquid crystal is aligned in a direction parallel to the electric field direction, the linearly-polarized light is passed through the liquid crystal layer without rotation, so it is blocked by the analyzer due to the vertically-polarized direction of analyzer, so not to be passed. Thereby, light may be selectively transmitted by controlling the liquid crystal. Thus, it is very important to uniformly align the liquid crystal throughout the whole LCD panel in order to provide uniform brightness and a high contrast ratio.
The conventional method of aligning the liquid crystal includes coating a polymer membrane such as a polyimide on a substrate made of a material such as glass, and rubbing the surface of the substrate with a fiber such as nylon or polyester in a predetermined direction. However, the rubbing method may cause serious problems while fabricating a liquid crystal panel due to fine dust or electrostatic discharge (ESD) that may be generated while rubbing the polymer membrane with the fiber.
Recently, there has been research on a photo-alignment method to cause a photo-crosslinking reaction and the like anisotropically by polarized photo-radiation rather than friction to thus bring about anisotropy on the surface of a polymer, and align liquid crystal molecules in one direction.
As polymer film materials for the photo-alignment method, polymers having photo-functional groups such as azobenzene, cumarine, chalcone, and cinnamate have been suggested. To prepare the polymers with photo-functional groups, dinitro-based compounds with excellent stability have been used. However, since double bonds where photo-crosslinking occurs may be broken down due to polarized photo-radiation in the course of preparing the polymers with photo-functional groups, it is difficult to use the dinitro-based compounds as a photo-alignment agent. In addition, the process of preparing the diamine including a photo-functional group can be too complicated to be economical.